img.borderColor{
    border: 1px solid red;
}
a.borderColor{
    border: 1px solid blue;
}
/*选择器举例*/
.contentBox{/*类*/
    border: 1px solid red;
    overflow: hidden;
    outline: 1px solid yellow;
    margin: 0px auto 50px;
}
.borderBox{
    margin-top: 50px;
    border: 1px solid blue;
}
#update{/*id*/
    color: blue;
}
div{/*标签*/
    border: 1px solid #ccc;
}
.contentBox,.borderBox{/*逗号*/
    width: 200px;
    height: 150px;
    padding: 50px;
}
figure figcaption{/*后代*/
    color: aqua;
}
/*伪类*/
a:link{ /*未触发前的状态*/
    color: green;
}
a:visited{ /*触发后的状态*/
    color: blueviolet;
}
a:hover{ /*鼠标悬浮时状态*/
    color: crimson;
}
a:active{ /*点击瞬间的状态*/
    color:yellowgreen;
}
/*伪元素*/
p:first-letter{
    font-size: 30px;
}
p:first-line{
    color: chocolate;
}
figure>p{
    border: 1px solid #ccc;
}
figure+p{
    background-color:cadetblue;
}
div~p{
    text-align: center;
}
[id=update]{
    border: 1px solid red;
}
[href~="#"]{
    background-color: burlywood;
}
ol li:first-child{
    color: red;
}
div :only-child{
    text-decoration: underline;
}
ul li:nth-child(2){
    color: yellow;
}